Even when compiling with -O0, GCC uses non-standard parameter passing conventions for static inline functions, which makes debugging difficult.

In this case, the function of interest converts a tagged value to a pointer; it's very helpful to be able to use this in GDB 'print' commands.

In the assembly code shown, notice that the function is expecting to find its argument in %eax, even though the standard IA-32 calling conventions pass the first argument on the stack.  The GDB session also demonstrates this.

$ (cd ~/gcc/gcc; svn info)
Path: .
URL: svn://gcc.gnu.org/svn/gcc/trunk
Repository Root: svn://gcc.gnu.org/svn/gcc
Repository UUID: 138bc75d-0d04-0410-961f-82ee72b054a4
Revision: 144927
Node Kind: directory
Schedule: normal
Last Changed Author: gccadmin
Last Changed Rev: 144927
Last Changed Date: 2009-03-17 17:16:40 -0700 (Tue, 17 Mar 2009)

$ cat jsobj.i
extern "C"
{
  static inline struct JSObject *JSVAL_TO_OBJECT (int v)
  {
    return (struct JSObject *) (v & ~3);
  }
}
static int
obj_toSource (int *vp)
{
  JSVAL_TO_OBJECT (vp[0]);
}
int
main(int argc, char **argv)
{
}
$ c++ -v -O0 -g jsobj.i -o jsobj -save-temps
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../gcc/configure --prefix=/home/jimb/gcc --enable-languages=c,c++
Thread model: posix
gcc version 4.4.0 20090318 (experimental) (GCC) 
COLLECT_GCC_OPTIONS='-v' '-O0' '-g' '-o' 'jsobj' '-save-temps' '-shared-libgcc' '-mtune=generic'
 /home/jimb/gcc/libexec/gcc/i686-pc-linux-gnu/4.4.0/cc1plus -fpreprocessed jsobj.i -quiet -dumpbase jsobj.i -mtune=generic -auxbase jsobj -g -O0 -version -o jsobj.s
GNU C++ (GCC) version 4.4.0 20090318 (experimental) (i686-pc-linux-gnu)
	compiled by GNU C version 4.4.0 20090318 (experimental), GMP version 4.2.2, MPFR version 2.3.2.
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
Compiler executable checksum: 00b593c3c99bbacc875575ff2b6655cc
COLLECT_GCC_OPTIONS='-v' '-O0' '-g' '-o' 'jsobj' '-save-temps' '-shared-libgcc' '-mtune=generic'
 as -V -Qy -o jsobj.o jsobj.s
GNU assembler version 2.18.93 (i486-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.18.93.20081009
COMPILER_PATH=/home/jimb/gcc/libexec/gcc/i686-pc-linux-gnu/4.4.0/:/home/jimb/gcc/libexec/gcc/i686-pc-linux-gnu/4.4.0/:/home/jimb/gcc/libexec/gcc/i686-pc-linux-gnu/:/home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0/:/home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/
LIBRARY_PATH=/home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0/:/home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-O0' '-g' '-o' 'jsobj' '-save-temps' '-shared-libgcc' '-mtune=generic'
 /home/jimb/gcc/libexec/gcc/i686-pc-linux-gnu/4.4.0/collect2 --eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o jsobj /usr/lib/crt1.o /usr/lib/crti.o /home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0/crtbegin.o -L/home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0 -L/home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0/../../.. jsobj.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0/crtend.o /usr/lib/crtn.o
$ cat jsobj.s
...
JSVAL_TO_OBJECT:
.LFB0:
	.file 1 "jsobj.i"
	.loc 1 4 0
	.cfi_startproc
	.cfi_personality 0x0,__gxx_personality_v0
	pushl	%ebp
	.cfi_def_cfa_offset 8
	movl	%esp, %ebp
	.cfi_offset 5, -8
	.cfi_def_cfa_register 5
	subl	$4, %esp
	movl	%eax, -4(%ebp)
	.loc 1 5 0
	movl	-4(%ebp), %eax
	andl	$-4, %eax
	.loc 1 6 0
	leave
	ret
...
$ gdb jsobj
GNU gdb (GDB) 6.8.50.20090106-cvs
Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
(gdb) start
Temporary breakpoint 1 at 0x804844c: file jsobj.i, line 16.
Starting program: /home/jimb/mc/b/static-call/jsobj 

Temporary breakpoint 1, main (argc=1, argv=0xbffff8a4) at jsobj.i:16
16	}
Current language:  auto; currently c++
(gdb) set $eax = 0x42424242
(gdb) print JSVAL_TO_OBJECT(0x53535353)
$1 = (struct JSObject *) 0x42424240
(gdb) quit
The program is running.  Quit anyway (and kill it)? (y or n) y
$